During use of a video camera or a digital still camera, in order to reproduce a white object as white, auto white balance adjustment is performed. In a well-known, conventional auto white balance method, the balance of RGB components (three primary color components of red, green, and blue) is adjusted so that the average of the whole image becomes achromatic. However, this method has a disadvantage in that incorrect white balance adjustment tends to be performed when a chromatic portion occupies the greater portion of an image.
This incorrect white balance adjustment is referred to as color failure. The technique disclosed in Patent Document 1 is known as an auto white balance adjustment method for reducing color failure. This technique divides an image into a plurality of blocks, calculates the average value of RGB for each block, and extracts only blocks in which the average value falls within a predetermined range. Then, RGB components are adjusted so that the average value of RGB of an extracted block group becomes achromatic.
Moreover, another auto white balance method for reducing color failure is disclosed in Japanese Patent Laid-Open Publication No. Hei 5-7369. This method avoids performance of excessive white balance adjustment by restricting a range of values which can be assumed by a white balance adjustment signal.
Although these methods are advantageous when a light source for illuminating an object is limited, sufficient balance adjustment cannot be performed when the object is illuminated by an unexpected light source or a plurality of light sources.
Therefore, in Japanese Patent Laid-Open Application Hei 8-289314 the present applicant has proposed an improved auto white balance adjusting apparatus. In this apparatus, the following block groups are extracted: a block group determined to be the result of dividing an image into a plurality of blocks and photographing a white object under fluorescent light, a block group determined to be the result of photographing a white object under daylight or tungsten light, and a block group having a color close to a brightest block in an image. Then, average values of each of RGB components on these block groups are obtained, and a value obtained by previously mixing the average values in accordance with a predetermined rule is used as a white balance adjustment signal. In this apparatus, by reflecting in a white balance signal the average value of RGB of a block group having a color close to the brightest block in an image, proper white balance adjustment can be performed even under an unexpected light source or a plurality of types of light sources.
However, because the apparatus disclosed in Japanese Patent Laid-Open Application Hei 8-289314 extracts the above block groups in accordance with only color, the value of a dark block is also used to obtain the average value of the block groups. A color appearing in an image is influenced by the color of an object and the color of a light source for illuminating the object. In this case, because a dark block often does not reflect the color of a light source, reflecting the information on the dark block in an auto-white-balance adjustment signal may lead to an error.
Therefore, in Japanese Patent Laid-Open Application 2000-92509, the present applicant has proposed a further-improved auto white balance adjusting apparatus. In this apparatus, the average value of RGB components obtained for each block obtained by dividing an image is used as the typical value of blocks; a block having the highest luminance is obtained in accordance with the typical value; and a luminance threshold value is obtained in accordance with the luminance of the highest-luminance block. Then, white balance adjustment is performed in accordance with the typical values of blocks having luminance higher than the luminance threshold value among all blocks.